1. Field of the Invention
The present invention relates to an optical device for splitting/combining a first and a second optical wavelength band.
Moreover, the present invention relates to an apparatus for splitting/combining a first and a second optical wavelength band, comprising at least two of said optical devices optically connected in cascade.
The present invention further relates to an optical unit and an optical network comprising at least one of said optical devices.
2. Description of the Related Art
In the present description and claims the expression                “transfer function” with reference to an output port and an input port of an optical device is used to indicate the ratio of the output optical power from the output port with respect to the total optical power in input to the input port of the optical device as a function of wavelength;        “passband”, with reference to an output port of an optical device, is used to indicate a band of wavelengths (or frequencies), between specified limits, that the output port is capable of transmitting; the limiting wavelengths of the passband are those at which the transmitted power level decreases to a specified level, typically 3 dB below the maximum level, as the wavelength is decreased or increased from that at which the transmitted power is a maximum;        “stopband”, with reference to an output port of an optical device, is used to indicate a band of wavelengths (or frequencies), between specified limits, that the output port is not capable of transmitting; the wavelengths of the stopband are those at which the transmitted power level is below a specified level, typically −20 dB below the maximum level of the corresponding passband at the same port;        “in-band loss” is used to indicate the difference (expressed in dB) between the maximum power level and the minimum power level of optical power transmitted by the port of a device within a predetermined optical wavelength (or frequencies) band which may correspond to the passband of the port of the device or to a sub-band thereof;        “isolation of a wavelength λa with respect to a wavelength λb” with reference to a port of a device is used to indicate the difference (expressed in dB) between the optical power transmitted at wavelength λb and the optical power transmitted at wavelength λ, wherein λa is a wavelength within the stopband of the port of the device and λb is a wavelength within the passband of said port;        “isolation of a wavelength band Ba with respect to a wavelength band Bb” with reference to a port of a device is used to indicate the difference (expressed in dB) between the minimum level of optical power transmitted within the band Bb and the maximum level of optical power transmitted within the band Ba, wherein the band Ba corresponds to the stopband of the port of the device or to a sub-band thereof and the band Bb corresponds to the passband of the port of the device or to a sub-band thereof;        “through port”, in a splitting/combining element having three optical paths and an input port associated with an end of one of the three optical path, is used to indicate the output port of the element which is associated to the opposed end of the optical path with which the input port is associated;        “cross port”, in a splitting/combining element having three optical paths and an input port associated with an end of one of the three optical path, is used to indicate the output port of the element which is associated to an end of an optical path other than the one with which the input port is associated;        “power splitting ratio” with reference to a splitting/combining element is used to indicate the ratio of the optical power exiting from each port of the element to the total optical power exiting from the splitting/combining element;        (25-50-25%)λx/(0-0-100%)λy three-optical-path splitting/combining element is used to indicate a device which comprises a first, a second and a third optical path, wherein the first optical path is optically coupled to the second optical path and the second optical path is optically coupled to the third optical path and wherein:        for a radiation at wavelength λx comprised in a first optical band in input to the first (third) optical path, is configured to let pass, at the output, about 25% of power of the radiation through the first (third) optical path, about 50% of the power from the first (third) optical path to the second optical path and about 25% of the power from the first (third) optical path to the third (first) optical path;        for a radiation at wavelength λy comprised in a second optical band in input to the first (third) optical path, is configured to let pass, at the output, about 100% of power of the radiation from the first (third) optical path to the third (first) optical path; and that        for a radiation at least at a wavelength λw comprised in the first optical band in input to the first (third) optical path, is configured to obtain at the output a phase difference Δφ of about +(π/2)+2K1π [or, −(π/2)+2K1π] between the radiation at the second optical path and the radiation at the first (third) optical path, and +π+2K2π (or, −π+2K2π) between the radiation at the third (first) optical path and the radiation at the first (third) optical path, where K1 and K2 are two integer numbers.        
It is pointed out that the above mentioned values of power splitting ratios and phase differences are to be intended to be equal to the cited values with a tolerance of about 20%, preferably 15%, on the power splitting ratio and 10%, preferably 5%, on the phase differences.
Currently, the research is turning towards the possibility of using optics in applications as fiber-to-the-premises (FTTP) networks for broadband delivering of voice, video and high-speed data directly to the home or to a broader community through optical fibers.
Converged voice, video and data services networks are also known as “triple play networks”. These networks typically support two signals in downstream direction (from a central station to a user) and one signal in upstream direction (from the user to the central station). Typically, one of the two downstream signals delivers analog television and the other downstream signal delivers digital voice and data services, such as for example telephone and/or Internet. The upstream signal is typically a digital signal delivering voice and data from the user to the central station. The downstream signal delivering analog television is typically transmitted in the wavelength band of 1550±10 nm, the other downstream signal is typically transmitted in the wavelength band of 1490±10 nm, and the upstream signal is typically transmitted in the wavelength band of 1310±50 nm. In order to provide a high-quality analog television service, power requirement in the wavelength band of 1550±10 nm is typically greatly demanding.
Typically, a FTTP network delivers voice, video and data over a passive optical network (PON) using the ITU-T gigabit passive optical network (GPON) standard.
In FTTP networks, as well in many other applications, a key issue is signal splitting and/or combining which allows the downstream and upstream signals to be suitably combined and/or splitted. Typically, the signal splitting/combining is performed in two steps: a first step for splitting/combining the two downstream signals from/with the upstream signal and a second step for splitting/combining the two downstream signals.
The first step requires the splitting/combining of two wide bands widely spaced (centred, for example, at around λ1=1310±50 nm and λ2=1520±40 nm).
This operation requires an optical band splitter/combiner device which, from an input radiation comprising two signals having wavelength within the two widely spaced wide bands, respectively, is capable of separating the two signals in two different ports of the device by optimizing at each port the loss of transmitted signal and the suppression of the other signal.
In particular, each of the two ports is required to have a wide and flat passband, corresponding to or including a respective one of the two widely spaced wide bands, and a wide stopband, corresponding to or including the other one of the two widely spaced wide bands, that provides a high isolation of the suppressed band with respect to the transmitted one.
Devices for splitting/combining two bands centred at 1300 nm and 1550 nm, respectively, have been proposed in the art.
For example, T. Kominato et al. (“Optical multi/demultiplexer with a modified Mach Zehnder interferometer configuration”, OEC '94, Technical Digest, July 1994, 14c2-4, pages 174-175) disclose a 1300/1550 nm multi/demultiplexer having a modified Mach-Zehnder (MZ) structure in which two 3 dB directional couplers of a conventional MZ are replaced with two MZ interferometers.
However, the Applicant notes that the replacement of the two 3 dB directional couplers with two MZ interferometers is disadvantageous for the compactness of the device.
G. Barbarossa et al. (“Wide rejection band multidemultiplexer at 1.3-1.55 mm by cascading high-silica three-waveguide couplers on Si”; Electronics Letters 24 Oct. 1991, Vol. 27, No. 22, pages 2085-2086) disclose a 1300/1550 nm multi/demultiplexer configuration obtained comprising three cascaded identical three-waveguide couplers connected by the two outer waveguides, wherein the guide separation and interaction length of the three-waveguide couplers are selected to obtain multi-demultiplexing at the two wavelengths 1300 and 1550 mm.